The application generally relates to a variable speed drive. The application relates more specifically to a variable speed drive with an active converter, and a control method for improved operating efficiency.
Variable speed drives (VSD) are commonly used for controlling the operating speed of synchronous and asynchronous motors. A VSD includes a converter for converting an AC line input voltage to a DC voltage, a DC link bus with a DC bus and capacitor storage and an inverter to provide variable AC output power to a motor load. The converter may be a passive or active converter. If an active converter is provided, certain benefits can be attained, such as controlling the operating power factor of the motor and associated equipment, and reducing harmonic noise.
The active converter typically includes semiconductor switches, such as IGBTs which switch currents to achieve low harmonic input current and DC bus voltage. The voltage rating of the DC bus is a function of the source input voltage to the VSD and the output voltage that the VSD provides to the motor. In commercial and industrial equipment, higher input and output voltage are required, and IGBT modules rated for 1700 Volts, or high voltage IGBT, are normally required to meet the desired output voltage to drive the motor. Other switches may also be used, including but not limited to MOSFETs, SiC MOSFETs, and GaN transistors. IGBTs rated for 1700 Volts, generate increased switching losses, resulting in reduced efficiency characteristics of the VSDs. By contrast, 1200V IGBT modules, or low voltage IGBT, are characterized by lower switching losses and increased efficiency, but are limited to a lower DC bus voltage.
Currently three different solutions are used to control VSDs. Some applications employ three levels of switches in the VSD converter, which requires twice the number of IGBTs when rated at lower voltages. Low voltage IGBTs may be used in that case, although this method increases the complexity and cost of the VSD. In another solution, a passive front end may be employed, however harmonic currents will be reflected back into the power system and require additional filters at the voltage source to meet the harmonic standards. Lastly, a third approach employs high voltage IGBT modules, resulting in higher losses and additional cost associated with the VSD.
The disclosure provides a method and system to reduce the DC bus voltage of a VSD to accommodate low voltage IGBTs to be used in the VSD while still achieving the DC link voltage sufficient to provide an increased AC voltage output from the inverter.
Intended advantages of the disclosed systems and/or methods satisfy one or more of these needs or provide other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.